[syslinux] Isohybrid wiki page and UEFI

[Thomas Schmitt]

Hi,

some minor nitpicking on Ady's summary:

> the real "genisoimage" is not really up to the task for 
> UEFI-booting.

In some distros, genisoimage offers option -e for embedding
a FAT filesystem image.
(genisoimage is a fork of mkisofs, so "real" is somewhat
 misleading in respect to history.)

For details see:
  http://www.syslinux.org/wiki/index.php/Isohybrid#UEFI

I am author of xorriso and willing to help with ISO 9660
composition, if questions arise.


> Alternatively, instead of using a "floppy-like" ("super-floppy", 
> "partition-less") FAT image, you "could" try a "HDD-like" image, with 
> GPT (somewhat equivalent to the "El Torito HDD-emulation" method under 
> BIOS)

I am not aware that EFI interprets the possibly present partition
table of the FAT image in the EFI System Partition.
(Regardless whether found by MBR partition table entry, GPT entry,
 or El Torito boot catalog entry.)

EFI from CD/DVD/BD has a soft size limit of 32 MB for its
System Partition.
That's due to the format of El Torito Boot Catalog which
counts the 512-bytes blocks by a 16-bit unsigned integer.

But one may attribute size 0 or 1, which means that the System
Partition reaches up to the end of the medium. (UEFI 2.4, 12.3.2.1)
There is no harm if actually other data are stored after the FAT
image.

This feature was still on my todo list. I took the occasion to
implement it in xorriso.
Now looking for testers with a real use case.


> Would anyone expect a UEFI-based hardware to boot from 
> an optical device in which the booting optical media is almost-all a 
> FAT32 HDD image?

In principle this should work.
But i saw rumors that attempts with SYSLINUX were not successful.
(Hard to say why failure happens with so many parameters involved.)

With BIOS, a similar method is used by a Seagate firmware installer
ISO image. Most of its software is in an El Torito floppy image
with a DR-DOS operating system on it. The ISO 9660 filesytem tree
is nearly empty.
So this ISO boots from CD into DR-DOS and (nearly ?) all further
processing stays in there. ISO 9660 is mainly used to make the
El Torito catalog visible to BIOS.


>  IMO, it would make more sense to just distribute a 
> (compressed) FAT image directly (no need for ISO9660) and use other 
> booting methods, such as a USB (flash) drive

This should be a viable method indeed, if not booting from optical
media is desired.


The Linux distros rather use a tiny GRUB2 FAT image with embedded
configuration (without any dialog of course) from where GRUB2
starts a search for the filesystem with the larger GRUB2
installation which finally offers a user visible menu.


Have a nice day :)

Thomas